The present invention pertains to radio transmission systems and more particularly to a method for weighting the quality of data transmitted and received by a radio transmission system.
Data systems that are expected to operate in noisy or lossy channels can employ error correction techniques to attempt to correct errors in the received signal. One such error correction technique is trellis coding, a method of convolutional coding. Trellis coding encodes a block of data by using the data as inputs to a finite state machine, where constellation point pairs are output according to state transitions. Fixed entry and exit states are used for the encoding state machine. The receiving equipment uses knowledge of the state machine to build a trellis diagram. The trellis diagram depicts traces through the least-cost choice for each state transition corresponding to each constellation pair. The lowest-cost path through the trellis diagram, beginning at the fixed entry and exit states, is chosen for decoding the data block. The Viterbi algorithm is used for finding the lowest-cost path.
One method for determining the least-cost path is by using a Hamming distance in a hard-decision decoder. The Hamming distance is then used as the metric for transition cost in the path. Since the hard decision discards much of the information about the signal, the hard decision method is not suitable for some systems. This method has losses corresponding to the quantization of received signal to bits.
It would be highly desirable to have a method which works as a kind of distance function and makes use of phase and magnitude information from the radio receiver. It is further desirable to strengthen the Viterbi search and have fewer quantization losses.